Spatial optical modulator

ABSTRACT

A spatial light modulator, in which light modulation elements are arranged in a same plane. In the spatial light modulator, a plurality of the light modulation elements are arranged such that there are at least two periods of periodic structure corresponding to an arrangement of the light modulation elements in an arbitrary direction in the plane where the light modulation elements are arranged.

FIELD OF THE INVENTION

The present invention relates to a spatial light modulator and the likewhich are used in a hologram recording and reproducing apparatus and thelike.

BACKGROUND ART

A volume holographic recording system is known as a digital informationrecording system using the principle of hologram. The feature of thissystem is to record an information signal in a recording medium asvariations in a refractive index. In the system, photorefractivematerial such as lithium niobate single crystal and the like is used forthe recording medium.

As one of hologram recording and reproducing methods, there is a methodfor recording and reproducing by use of Fourier transform.

FIG. 1 shows an example of a conventional hologram recording andreproducing apparatus. In this drawing, laser light 12 emitted from alaser light source 11 is divided into a signal light 12A and a recordingreference light 12B by a beam splitter 13. The beam diameter of thesignal light 12A is magnified by a beam expander 14, and the signallight 12A is applied to a spatial light modulator (SLM) 15 such as apanel of a translucent TFT liquid crystal display (LCD) and the like ascollimated light. The spatial light modulator (SLM) 15 receivesrecording data converted by an encoder 25 as an electric signal, to forma bright and dark dot pattern on a plane. In passing through the spatiallight modulator (SLM) 15, the signal light 12A is modulated to include adata signal component. When the signal light 12A including the signalcomponent of the dot pattern passes through a Fourier transform lens 16,which is disposed a focal length “f” away, the signal component of thedot pattern is subjected to Fourier transform, and is condensed into therecording medium 5.

On the other hand, the recording reference light 12B divided by the beamsplitter 13 is led into the recording medium (volume holographic memory)5 by a mirror 18 and a mirror 19. The recording reference light 12Bintersects with an optical path of the signal light 12A inside therecording medium 5 and forms a light-interference pattern, to record thewhole light-interference pattern as variations in a refractive index.

The Fourier transform lens, as described above, forms an image fromdiffracted light of image data, which is illuminated by coherentcollimated light. The image is converted into distribution on a focalplane, that is, on a Fourier plane, and the distribution as a result ofFourier transform is allowed to interfere with the coherent referencelight, in order to record interference fringes on the recording mediumin the vicinity of a focal point. After completing the recording of asingle data page (hereinafter, also simply referred to as a “page”), themirror 19 is rotated at a predetermined angle, and the position thereofis moved in parallel by a predetermined amount, in order to vary anincident angle of the recording reference light 12B with respect to therecording medium 5. Then, the second page is recorded in the sameprocedure. Angular multiplexing recording is carried out by successivelyperforming the recording like this.

In reproducing operation, on the other hand, inverse Fourier transformis carried out to reproduce a dot pattern image. In reproducing data, asshown in FIG. 1, the optical path of the signal light 12A is interruptedby, for example, the spatial light modulator (SLM) 15, and only thereference light 12B is applied to the recording medium 5. Duringreproduction, the position and angle of the mirror 19 are varied andcontrolled with the use of the combination of the rotation and linearmovement of the mirror 19 so that the incident angle of the recordingreference light becomes the same as that in recording a page to bereproduced. Reproduction light which reproduces the recordedlight-interference pattern appears on the opposite side of the recordingmedium 5 irradiated with the reference light 12B. A dot pattern signalcan be reproduced by leading the reproduction light into an inverseFourier transform lens 16A to carry out inverse Fourier transform. Then,the dot pattern signal is received by a photodetector 20 such as acharge-coupled device CCD and the like in the position of a focallength, to reconvert the dot pattern signal into an electric digitaldata signal. Then, the digital data signal is sent to a decoder 26, sothat original data is reproduced.

In the hologram recording with Fourier transform, the first-orderdiffracted light becomes the highest frequency component of the signallight, which is Fourier transformed by the spatial light modulator 15such as the LCD and the like, due to the repeats of pixels of thespatial light modulator 15.

FIG. 2 is a plan view showing a pattern of the conventional spatiallight modulator 15. Square pixels a single side of which has a length of“a” (μm) are arranged in a matrix. In other words, a pixel pitch of thespatial light modulator 15 is “a” (μm). The reference numeral 6indicates an incident beam which is incident on the spatial lightmodulator 15.

Referring to FIG. 3, the optical axis of the signal light represents a Zdirection, and the directions of columns and rows of the pixels in aplane perpendicular to the signal light represent X and Y directions,respectively. When the signal light interferes with the reference lightto record inside the recording medium 5, light intensity distributionsof spatial frequency spectrum occur in the XY plane, which is inparallel with the Fourier plane, symmetrically with respect to theoptical axis of the signal light.

The hologram recording using a Fourier transform hologram has theadvantages that hologram fits into spatially limited space, informationis recorded in a distributed manner by use of Fourier transform, and theredundancy of recording can be increased. The distance (d1) between azero-order Fourier spectrum and the first-order Fourier spectrum in theFourier plane is expressed as follows, with the use of a spatialfrequency (fsp) in a recording plane, the wavelength (λ) of light, andthe focal length (F1) of the Fourier transform lens.d1=fsp•λ•F1

Since the pixel pitch of the spatial light modulator 15 is 42 μm, thewavelength is 532 nm, and the focal length is 165 mm, the Fourierspectrum distance (d1) of the corresponding highest frequency componentis 2.1 mm, according to the foregoing equation. Thus, information to berecorded exists in a range of approximately ±2.1 mm on the optical axis.In other words, as shown in FIG. 3, two-dimensional data appearing inthe spatial light modulator 15 is distributed over xy space (x, y≦±2d1)in a matrix with two rows and two columns, which is composed of thefirst-order diffracted light and zero-order light.

Therefore, a peak appears in a Fourier transformed image of the spatiallight modulator 15, in accordance with the highest frequency componentdue to the pixel pitch. These peaks themselves do not bear anymeaningful data. If these peaks occur in such a Fourier transformedimage, the photorefractive effect of the recording medium becomessaturated in the above-mentioned peak position, so that there is aproblem that nonlinear distortion tends to occur in a recorded image.

Also there is a method for offsetting the recording medium from theFourier plane in order to secure a dynamic range during recording, butthe method has the problems that time necessary for recording becomeslong, an S/N ratio decreases, highly sensitive recording medium isneeded, and the like.

Considering the foregoing problems, an object to be achieved by thepresent invention includes one example of the foregoing problems. Inother words, an object of the present invention is to provide a spatiallight modulator with high performance which can record with highsensitivity and less signal distortion.

SUMMARY OF THE INVENTION

In a spatial light modulator according to the present invention, aplurality of light modulation elements are arranged in one plane. Theplurality of light modulation elements are arranged so that there are atleast two periods of periodic structure corresponding to the arrangementof the light modulation elements in an arbitrary direction in the plane.

In a spatial light modulator according to the present invention, aplurality of light modulation elements are arranged in a circular lightmodulation region. The plurality of light modulation elements arearranged so that there are at least two periods of periodic structurecorresponding to the arrangement of the light modulation elements in anarbitrary direction in the light modulation region. The size of thelight modulation element increases along the outer peripheral directionof the light modulation region.

A spatial light modulator according to the present invention has acircular light modulation region. A light modulation element is disposedin each of areas which are obtained by radially and concentricallydividing the light modulation region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a conventional hologramrecording and reproducing apparatus;

FIG. 2 is a plan view showing a pattern of a conventional spatial lightmodulator in which square pixels, a side of which has a length of “a”,are arranged in a matrix;

FIG. 3 is a diagram showing a light intensity of frequency spectrum,which occurs in an xy plane in parallel with a Fourier plane due to theinterference between signal light and reference light;

FIG. 4 is a block diagram showing the structure of a hologram recordingand reproducing apparatus which uses a spatial light modulator accordingto a first embodiment of the present invention;

FIG. 5 is a schematic plan view showing the shapes of light modulationelements in the spatial light modulator according to the firstembodiment of the present invention;

FIG. 6 is a partly enlarged view of the spatial light modulator shown inFIG. 5;

FIG. 7 is a schematic plan view showing the structure of a spatial lightmodulator according to a second embodiment of the present invention;

FIG. 8 is a schematic plan view showing the structure of a spatial lightmodulator with circular light modulation elements according to anotherembodiment of the present invention; and

FIG. 9 is a schematic plan view showing the structure of a spatial lightmodulator with rectangular light modulation elements according tofurther another embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail withreference to the drawings. In the drawings described below,substantially identical parts are denoted by the same referencenumerals.

FIRST EMBODIMENT

FIG. 4 is a block diagram showing the structure of a hologram recordingand reproducing apparatus 10 which uses a spatial light modulator 40according to a first embodiment of the present invention.

In an optical system 10A of the hologram recording and reproducingapparatus 10, for example, a solid-state laser for emitting green lightwith a wavelength of 532 nm is used as a light source of signal light12A and recording reference light 12B. A laser light source 11 is drivenby a laser driver 31. The laser driver 31 is controlled by a maincontroller (CPU) 30 connected to each circuit block of the hologramrecording and reproducing apparatus 10 to carry out the control of thewhole apparatus. More specifically, various control signals including awrite timing signal and the like is supplied from the main controller 30to the laser driver 31, and the laser driver 31 drives the laser lightsource 11 on the basis of the control signals.

Laser light 12 emitted from the laser light source 11 is divided intothe signal light 12A and the recording reference light 12B by a beamsplitter 13. A beam expander 14 magnifies the beam diameter of thesignal light 12A, and the signal light 12A is incident on the spatiallight modulator (SLM) 40, which comprises a panel of a translucent TFTliquid crystal display (LCD), as collimated light.

A plurality of light modulation elements are arranged in the spatiallight modulator (SLM) 40, in such a manner that there are at least twoperiods of periodic structure which corresponds to the arrangement ofthe light modulation elements in an arbitrary direction in a plane. Inother words, the light modulation elements are arranged so that thereare at least two peak components of a Fourier frequency in a Fourierplane corresponding to the arrangement of the light modulation elements.

In this embodiment, as shown in a plan view of FIG. 5, the spatial lightmodulator (SLM) 40 has a circular light modulation region or area 6Awhich is approximately inscribed with the beam diameter 6 of the signallight. The light modulation region 6A is divided every predeterminedangle (θ) by radial partition lines passing through the center of thecircle. The light modulation region 6A is also concentrically divided bypartition lines the radiuses of which are R₁, R₂, . . . , R_(n). Eachdivided area corresponds to a light modulation element (hereinafter,also referred to as pixel) 40A, and hence the spatial light modulator 40comprises pixels A_(k,1), A_(k,2), . . . , A_(k,n) (k=1, 2, . . . , m).Therefore, the spatial light modulator 40 comprises n×m pixels.

Taking a case of k=1, for example, as shown in a partly enlarged view ofFIG. 6, each of the pixels A_(1,1), A_(1,2), . . . , A_(1,n) isconfigured so as to have a different pitch in a radial direction. It ispreferable that no pixel has the same pitch. If the pixels areconfigured so that the reciprocals of the pitches are the approximatelysame values, the distance between Fourier spectra can be evenlydistributed. By configuring the spatial light modulator 40 in thismanner, the distance between Fourier spectra each of which correspondsto each pixel differs in the Fourier plane, so that it is possible toprevent a peak from occurring in a specific position in a Fouriertransformed image.

Furthermore, in this embodiment, pixels with high spatial frequencies,i.e., small pixels are arranged in a central portion, and pixels withlow spatial frequencies, i.e., large pixels are arranged in a peripheralportion, in order to effectively obtain an amount of incident light on alens. In other words, the length L_(1,j) (j=1, 2, . . . , n) of eachpixel A_(1,1), A_(1,2), . . . , A_(1,n), in a radial direction becomeslong as the pixel approaches the peripheral portion (as j increases).

Furthermore, the size of each pixel A_(1,1), A_(1,2), . . . , A_(1,n) isdetermined in accordance with the power density of a signal light beam.In other words, when the signal light beam has the shape of Gaussiandistribution, the power density is high in the central portion of thebeam, and decreases with approaching the peripheral portion of the beam.Thus, the size of each pixel is determined so that the power of lightincident on each pixel becomes substantially equal. The size of eachpixel may be determined so that a ratio of the power of light incidenton each pixel is within a predetermined range.

The spatial light modulator (SLM) 40 forms a bright and dark pattern onthe basis of a data signal to be recorded. To be more specific, anencoder 25 receives the recording data signal which comprises aone-dimensional digital signal sequence, to convert the signal into atwo-dimensional data array in accordance with the pixel array of theforegoing spatial light modulator (SLM) 40. Furthermore, the encoder 25adds an error correction code to the two-dimensional data array, andgenerates a two-dimensional data signal (a unit page series datasignal). The encoder 25 is provided with an SLM driver (notillustrated). The SLM driver generates a driving signal on the basis ofthe two-dimensional data signal, to drive the spatial light modulator(SLM) 40. Therefore, a two-dimensional pattern is formed in the spatiallight modulator (SLM) 40 in accordance with the two-dimensional datasignal.

When the signal light 12A passes through the spatial light modulator(SLM) 40, it is subjected to light modulation with the pattern. In otherwords, the spatial light modulator 40 has a modulation processing unitwhich corresponds to a unit page. The spatial light modulator 40 turnson or off the light of an applied coherent signal beam with a wavelengthof 532 nm pixel-by-pixel, in accordance with unit page series data fromthe encoder 25, in order to generate a modulated signal light beam. Tobe more specific, the spatial light modulator 40 passes the signal beamwhen a logical value of unit page series data, being an electric signal,is “1”, and intercepts the signal beam when the logical value is “0.”Thus, electro-optic conversion is accomplished in accordance with thecontents of each bit in unit page data, and hence the modulated signallight beam (signal beam) is generated as signal light of a unit pageseries.

The signal light 12A including the recording data signal passes througha Fourier transform lens 16 which is disposed a focal length “f” away,and the pattern signal component is subjected to Fourier transform to becondensed into the recording medium 5.

The recording reference light 12B divided through the beam splitter 13,on the other hand, is led into a recording medium (volume holographicmemory) 5 by a mirror 18 and a mirror 19. The recording reference light12B intersects with an optical path of the signal light 12A inside therecording medium 5 and forms a light-interference pattern, to record thewhole light-interference pattern as variations in a refractive index.

The Fourier transform lens, as described above, forms an image fromdiffracted light from the spatial light modulator, which is illuminatedby coherent light and modulated with image data. The image interfereswith the coherent reference light, and interference patterns arerecorded in the recording medium in the vicinity of a focal point. Uponcompleting the recording of a single data page (hereinafter, also simplyreferred to as “page”), a recording medium driver 33 moves the positionof the recording medium 5 in parallel by a predetermined amount, and thesecond page is recorded in the same procedure. Recording is carried outby successive recording like this.

In reproducing, on the other hand, the image is reproduced by carryingout inverse Fourier transform. In the reproduction of data, for exampleas shown in FIG. 4, after the recording medium driver 33 moves therecording medium 5 to a predetermined position, a shutter 17 or thespatial light modulator (SLM) 40 interrupts the optical path of thesignal light 12A, so that only the reference light 12B is incident inthe recording medium 5. Thus, reproduction light is reproduced from therecorded light-interference pattern. A pattern signal is reproduced byleading the reproduction light into an inverse Fourier transform lens16A and carrying out inverse Fourier transform. Then, after the patternsignal received by a photodetector 20 such as a charge-coupled device(CCD) and the like is reconverted into an electric digital data signal,the digital data signal is sent to the decoder 26 in order to reproducerecorded data.

According to the present invention, it is possible to reduce theintensity of the light peak occurring in the Fourier transformed image,which is caused by the periodic structure of the spatial lightmodulator. Thus, it is possible to prevent the photorefractive effect ofthe recording medium from being saturated. Therefore, it is possible toprovide a spatial light modulator with high performance, by whichnonlinear distortion is hard to occur and hologram recording is carriedout with high sensitivity.

SECOND EMBODIMENT

FIG. 7 is a schematic plan view showing the structure of a spatial lightmodulator 41 according to a second embodiment of the present invention.The spatial light modulator 41 comprises a panel of a transmission-typeTFT liquid crystal display (LCD).

As shown in the plan view of FIG. 7, the spatial light modulator (SLM)41 comprises a plurality of light modulation elements (pixels) 41A, eachof which has the shape of a circle. The plurality of pixels 41A arearranged in the plane of the spatial light modulator 41, in such amanner as to satisfy any one of the following conditions.

-   (1) The pixels 41A are arranged so that spatial frequencies by the    pixels 41A in an arbitrary line in the plane of the spatial light    modulator 41 become plural, or-   (2) The plurality of pixels 41A have random sizes, or-   (3) The pixel with a high spatial frequency, i.e., the small pixel    is arranged in a central portion, and the pixel with a low spatial    frequency, i.e., the large pixel is arranged in a peripheral    portion.

The pixels may be arranged so as to satisfy a plurality of the foregoingconditions. In this embodiment, the pixels are arranged so as to satisfyall of the foregoing conditions (1) to (3).

It is preferable that the size of each pixel is determined in accordancewith the power density of a signal light beam. Namely, when the signallight beam density has the shape of Gaussian distribution, the powerdensity is high in the central portion of the beam, and decreases withapproaching the peripheral portion of the beam. Therefore, the size ofeach pixel may be determined so that a ratio of the power of lightincident on each pixel is within a predetermined range.

FIG. 8 is a schematic plan view showing another embodiment of thespatial light modulator 41 according to the present invention. Eachlight modulation element (pixel) 41A has the shape of a circle, and thesize of each pixel increases toward the outer peripheral direction ofthe light modulation region 6A.

Each pixel does not need to be in the shape of a circle. As shown inFIG. 9, a spatial light modulator 42 may comprise rectangular pixels42A. In this embodiment, the pixels 42A are arranged so as to satisfyall of the foregoing conditions (1) to (3).

Furthermore, every pixel does not need to be in the same shape. In otherwords, it is unnecessary that all pixels have the shape of a circle or arectangle, and pixels in random shapes may be arranged.

According to such a structure, since light peak intensity occurring inthe Fourier transformed image is reduced, it is possible to prevent thephotorefractive effect of the recording medium from being saturated.Therefore, it is possible to provide a spatial light modulator with highperformance, by which nonlinear distortion is hard to occur and hologramrecording is carried out with high sensitivity.

1. A spatial light modulator for use in hologram recording, in which a plurality of light modulation elements are arranged in one plane to modulate a light beam incident thereon, wherein: said plurality of light modulation elements are configured to perform light modulation in accordance with two-dimensional data corresponding to a recording data signal, and said plurality of light modulation elements are arranged such that there are at least two Fourier frequency components corresponding to distances of the light modulation elements in an arbitrary direction in said one plane.
 2. The spatial light modulator according to claim 1, wherein said plurality of light modulation elements perform light intensity modulation.
 3. A spatial light modulator for use in hologram recording, in which a plurality of light modulation elements are arranged in a light modulation region of a circular shape to modulate a light beam incident thereon, wherein: said plurality of light modulation elements are configured to perform light modulation in accordance with two-dimensional data corresponding to a recording data signal, and said plurality of light modulation elements are arranged such that there arc at least two Fourier frequency components corresponding to distances of the light modulation elements in an arbitrary direction in said light modulation region, and sizes of the light modulation elements increases along an outer peripheral direction of said light modulation region.
 4. The spatial light modulator according to claim 3, wherein said plurality of light modulation elements have areas such that the ratios of light powers incident on the respective light modulation elements fall within a predetermined range.
 5. The spatial light modulator according to claim 3, wherein said plurality of light modulation elements perform light intensity modulation.
 6. A spatial light modulator for use in hologram recording and having a light modulation region of a circular shape to modulate a light beam incident thereon, comprising: a plurality of light modulation elements arranged in areas which are obtained by radially and concentrically dividing said light modulation region, said plurality of light modulation elements being configured to perform light modulation in accordance with two-dimensional data corresponding to a recording data signal, wherein said plurality of light modulation elements are positioned such that there are at least two Fourier frequency components corresponding to distances of the light modulation elements in a radial direction of said light modulation region.
 7. The spatial light modulator according to claim 6, wherein said plurality of light modulation elements have areas such that the ratios of light powers incident on the respective light modulation elements fall within a predetermined range.
 8. The spatial light modulator according to claim 6, wherein said plurality of light modulation elements perform light intensity modulation. 